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Hunt for cold H 2 molecules

Hunt for cold H 2 molecules. Françoise Combes, Observatoire de Paris 20 Septembre 2005. H 2 pure rotational lines. Uncertainties in X= H 2 /CO.  Metallicity gradients (factor 10)  Density structure (factor 10, across the mass spectrum)

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Hunt for cold H 2 molecules

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  1. Hunt for cold H2 molecules Françoise Combes, Observatoire de Paris 20 Septembre 2005

  2. H2 pure rotational lines

  3. Uncertainties in X= H2/CO Metallicity gradients (factor 10) Density structure (factor 10, across the mass spectrum) Excitation conditions: High-z SB galaxies, X divided by 5 In spiral and dwarf galaxies, rotation curves could be explained through HI-scaling by a factor 7-10 Quite easy to explain rotation curves with dark H2 gas Only 10-20% of the dark baryons in galaxies The rest in cosmic filaments Dark baryons (90% of them) are not in compact objects (microlensing) and should be in gas: either hot or cold

  4. Perspectives of detection H2 is a furtive molecule at low T: symmetric, no dipole Hyperfine structure? (or ultrafine) Para-H2: I=0, J even -- Ortho-H2: I=1, Jodd F=I+J  F = 0, 1 and 2 Interaction between nuclear spin, and B generated by rotation F=1-0: 546.4 kHz (l =0.5km) F=2-1 54.8 kHz (l= 5.5km) Line intensities extremely weak (nuclear magnetic dipole, A = 10-32s-1) Require 106x surface for HI detection, i.e. 300x300km (beam 5’) Grid at l/4, 8 per km (2400x2400 grid on the Moon?)

  5. At the limit of T=0, H2 should be all para • O/P = 9 exp(-170/T) • But out of equilibrium, at low density • Proton exchange H+ + H2(J=1)  H+ +H2(J=0) • Significant amount of ortho-H2 at 3K At 3K, the pressure of H2 clumps is 100 times the saturated vapor pressure (Combes & Pfenniger 1997) Latent heat = 110K/H2, no time to form much snow But conditions for dimerisation Continuum emission of dimers (dipole induced by collisions) Schaefer (1996, 1998)

  6. Primordial molecules HD: weak dipole moment (proton more mobile than deuteron) m= 5.8 10-4 Debye(Trefler & Gush 1968) J=1-0, 130 K above ground state, l = 112m Only from excited regions  not a good tracer LiH: m= 5.9 Debye(Laurence et al 1963) J=1-0, 21 K above ground state, l = 0.67mm (450GHz) Impossible from the ground, H2O absorption LiH/H2 ~ 10-10 t ~1 N(LiH) = 1012cm-2 or N(H2) =1022cm-2

  7. H2+: Abundance 10-11-10-10 Hyperfine structure, but not in N=0 state, only N=1, I=1, S=1/2 Eu=110K, the more likely at 1343 MHz Traces of C and O in « primordial » gas A residual 10-3 solar abundance in Lya clouds CO emission very sensitive to Z (t=1 cloud size varies) Threshold of photo-dissociation (Av=0.25mag?) Problem of heating sources, T ~ Tbg Absorption, bias towards diffuse clouds Surface filling factor < 1%

  8. Hardly visible cold H2 Clouds Mass ~ 10-3 Mo density ~1010 cm-3 size ~ 20 AU N(H2) ~ 1025 cm-2 tff ~ 1000 yr Adiabatic regime: much longer life-time Fractal: collisions lead to coalescence, heating, and to a statistical equilibrium (Pfenniger & Combes 94) Around galaxies, the baryonic matter dominates Connection with the filaments The stability of cold H2 gas is due to its fractal structure

  9. Fractal M ~ rD Recursive Jeans fragmentation Projected mass log scale (15 mag) The surface filling factor depends strongly on D < 1% for D=1.7 Pfenniger & Combes 1994

  10. g-rays: Dark gas in the solar neighborhood Dust detected in B-V (by extinction) and in emission at 3mm g-Emission associated to the dark gas H2 x a factor 2 (or more) Grenier et al (2005)

  11. Cooling flows in galaxy clusters Cooling time < Hubble time at the center of clusters  Gas Flow, 100 to 1000 Mo/yr Mystery:cold gas or stars formed are not detected? Today, the amplitude of the flow has been reduced by 10 and the cold gas is detected Edge (2001) Salomé & Combes (2003) 23 detected galaxies in CO Results from Chandra & XMM: cooling flow self-regulated Re-heating process, feedback due to the active nucleus or black Hole: schocks, jets, acoustic waves, bubles...

  12. Perseus Ha (WIYN) and CO (IRAM) Salome, Combes, Edge et al 05 Ha, Conselice 01

  13. Perseus Cluster Fabian et al 2003

  14. OSER: Optical Scintillation by Extraterrestrial Refractors Moniez, 2005

  15. ISO –Pure H2 rotational lines N(H2)= 1023 cm-2 T = 80 – 90 K 5-15 X N(HI) NGC 891 Valentijn & Van der Werf 99

  16. H2EXplorer • 4 lines • 1000 x more sensitive ISO-SWS • L2 • Soyuz • 99 Meuro Survey integration 5s limit total area [sec] [erg s-1 cm-2 sr-1] [degrees] Milky Way 100 10-6 110 ISM SF 100 10-6 55 Nearby Galaxies 200 7 10-7 55 Deep Extra-Galactic 1000 3 10-7 5  CNES  Spitzer  Milky Way, NGC 1560

  17. Conclusion The H2/CO conversion ratio is still a big question in many « non-standard » circumstances: high-redshift galaxies, external regions of galaxies, etc.. There could be large quantities of cold H2 not yet detected Best future tracers  pure rotational H2 lines, with H2* excited as tracer (H2EX)  gamma rays (but cosmic rays?)  primordial molecules, HD, LiH (high z, ALMA)  absorption lines (UV, mm..)  continuum dust emission (e.g. Miville-Deschênes et al 2005)

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